Reduced Agulhas Leakage during the Last Glacial Maximum inferred from an integrated provenance and flux study

etters 250 (2006) 72–88www.elsevier.com/locate/epsl

Earth and Planetary Science L

Reduced Agulhas Leakage during the Last Glacial Maximuminferred from an integrated provenance and flux study

Allison M. Franzese ⁎, Sidney R. Hemming, Steven L. Goldstein, Robert F. Anderson

Lamont-Doherty Earth Observatory, Department of Earth and Environmental Sciences, Columbia University, Palisades, NY 10964, USA

Received 1 March 2006; received in revised form 30 June 2006; accepted 6 July 2006Available online 7 September 2006

Editor: M.L. Delaney

Abstract

Surface and intermediate waters from the Indian Ocean enter the Cape Basin in the southeast Atlantic by the “AgulhasLeakage”, which adds heat and salt to the Atlantic Ocean, and may act as a positive feedback for the formation of North AtlanticDeep Water (NADW). In order to assess the role of the Agulhas Leakage in past climate change, it is important to constrainwhether there was change in its flux in association with warmer and colder global climate intervals. This study uses the radiogenicisotope compositions of strontium and neodymium and the rubidium, strontium, samarium and neodymium concentrations of theterrigenous fraction of sediments from the oceans surrounding South Africa as tracers of sediment provenance, and the initialexcess Thorium-230 of the bulk sediments as a constant flux proxy. The purpose is to assess the relationship between sedimentsources around southern Africa and Agulhas Current flow for the Holocene and Last Glacial Maximum (LGM). Results point to theAgulhas Current as a major source of sediment to the southern Cape Basin during the Holocene, and show that the surface currentshave an important control on the distribution of sediments in the South Atlantic. The Cape Basin data can be explained with threeend-member mixing of particulates carried by (1) the Agulhas Current, (2) the South Atlantic or Antarctic Circumpolar Current(SAC or ACC) and (3) locally derived sediments from southern Africa. The composition of each end-member does not changesignificantly between the LGM and the Holocene. Comparison of the two time-slices show that a smaller fraction of sedimentdeposited in the Cape Basin is derived from the Agulhas Current during the LGM. The ACC and SAC were more sediment ladenduring the LGM and western sources were major contributors to the Cape Basin sediments. The data also indicate a much reducedAgulhas contribution to sediments deposited south and southeast of Africa beneath the present-day Agulhas Current flow,indicating a reduced transport of the Agulhas Current and reduced Agulhas Leakage during the LGM.© 2006 Elsevier B.V. All rights reserved.

Keywords: Agulhas Current; Agulhas Leakage; provenance; strontium isotopes; neodymium isotopes; Th-normalization; constant flux proxy;paleoceanography

⁎ Corresponding author.E-mail address: [email protected] (A.M. Franzese).

0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.07.002

1. Introduction

For nearly two decades it has been recognized thatthe strength of thermohaline circulation, particularlychanges in North Atlantic Deep Water (NADW)formation, is an important driving force and/or a

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http://dx.doi.org/10.1016/j.epsl.2006.07.002

73A.M. Franzese et al. / Earth and Planetary Science Letters 250 (2006) 72–88

major amplifier for the climatic cycles of the LateQuaternary [1]. In today's ocean, the surface waters ofthe Atlantic Ocean sink to form NADW, representingthe beginning of the “Global Conveyor Belt” thattransports heat and salt through the world's oceans. Oneof the two routes for returning shallow water to theNorth Atlantic is around the tip of Africa via theAgulhas Current, where Indian Ocean water “leaks”into the Atlantic (e.g. [2–4]). The Agulhas Current isthe largest western boundary current in the world ocean,with an average transport of ∼65 Sv [4]. It transportssurface and intermediate water from the tropical IndianOcean southward along the coast of Africa, until itreaches the tip of Africa and detaches from the coast.Here, the current turns back on itself, and flowseastward as the Agulhas Return Current, parallel withthe Antarctic Circumpolar Current (ACC) (Fig. 1). Atthis zone of Retroflection, warm-core eddies andfilaments are shed into the South Atlantic as the“Agulhas Leakage”. These eddies and filaments canbecome entrained in the northward-flowing Benguela

Fig. 1. Study area, core locations and a schematic representation of the oceancores located under the influence of the Agulhas Current are shown as black dis shown as a black triangle. Cores in the Cape Basin are shown using black sare shown as black circles and those cores under the South Atlantic Curridentification and comparison with subsequent Tables and Figures. Surface cuARC = Agulhas Return Current; BC = Benguela Current; SAC = South Acirculation [58] is shown by thin black arrows: NADW = North Atlantic DeBottom Water.

Current, and ultimately contribute heat and salt to theNorth Atlantic Ocean. This Indian–Atlantic exchangemay be an important feedback mechanism for theformation of NADW, as originally proposed by Gordonand colleagues [2,3].

Subsequent modeling studies and observations gen-erally agree that the Agulhas Leakage does have someeffect on the global stratification and thermohalineoverturning circulation [4]. The mechanisms of stimu-lating NADW formation may include not only theadvection of heat and salt to the high latitude NorthAtlantic, but also the meridional density gradients thatthey create within the Atlantic Ocean [5]. Recent modelresults [6] support Gordon's hypothesis, showing thatsalt input from the Agulhas Leakage preconditionsNADW formation and continued leakage and salinityadvection act as a positive feedback for the thermohalinecirculation. For this reason, many studies have tried toassess the variability in the Agulhas Leakage ondifferent timescales [7–12] and relate it to NorthAtlantic climate variability.

circulation for the Cape Basin and surrounding oceans. The deep-seaiamonds. The core on the Tugela Cone, also under the Agulhas Current,quares. Cores under the influence of the Antarctic Circumpolar Currentent are grey circles. The cores are numbered 1 through 13 for easyrrents [56,57] are shown as thick, light arrows: AC = Agulhas Current;tlantic Current; ACC = Antarctic Circumpolar Current. Deep waterep Water; AAIW = Antarctic Intermediate Water; AABW = Antarctic

74 A.M. Franzese et al. / Earth and Planetary Science Letters 250 (2006) 72–88

This study follows from previous high resolutionwork on deep-sea core RC11-83 [13–20] (41.07° S,9.717° E, 4718 m, Fig. 1, sedimentation rate ∼25 cm/ka) retrieved from a site located on a sediment drift inthe southeastern Cape Basin. Major deep water massesin the Cape Basin include northward flowing AntarcticIntermediate Water (AAIW) and Antarctic BottomWater (AABW), which sandwich the southward flowingNADW. NADW enters the Cape Basin through a deep(∼3500 m) passage in the Walvis Ridge to the north,while AABW enters from the southeast, between theAgulhas Ridge and South Africa (Fig. 1). The bottomwaters flow clockwise within the basin, formingcontourite deposits such as the one from which coreRC11-83 was taken. Several records from RC11-83have been interpreted to reflect changes in the relativeproportions of northern- and southern-source waters,

Fig. 2. Downcore records from RC11-83 (41.07° S, 9.717° E, 4718 m): The HHolocene. Terrigenous 87Sr/86Sr [14] show systematic variations on glacial–strontium was delivered via the Agulhas Current, and inferred that the glaciawater entering the South Atlantic. Benthic δ13C [13] and authigenic εNd [19] wsite. NADW varies on glacial–interglacial timescales, with higher δ13C value

implying changes in the strength of global thermohalinecirculation [13,14,18,19,21]. A record of the strontiumisotope ratios (87Sr/86Sr) in the terrigenous fraction ofthe sediment shows temporal variability that matchesthe pattern of the circulation proxies (Fig. 2), and thishas been discussed in the context of changes in theAgulhas Leakage [14]. More recently, noting the lack ofcorrelation between measures of sea surface temperatureand 87Sr/86Sr, this same record has been discussed interms of changes in the amount of eolian dust fromPatagonia reaching the site [20].

To address the question of whether the terrigenousdetrital fraction of sediments around southern Africaindicate reduced Agulhas Leakage during the LastGlacial Maximum (LGM, ∼20 ka), we carried out thefirst study that combines radiogenic isotopes asprovenance tracers along with initial excess Thorium-

olocene and LGM are labeled and the shaded region indicates the warminterglacial timescales. The authors hypothesized that the radiogenicl–interglacial variations are due to variations in the amount of Agulhasere interpreted to reflect the relative proportion of NADW reaching thes and lower εNd (indicating more NADW) during interglacial periods.


230 (230Thxs0 ) as a constant flux proxy for a suite of

sediment cores from the South Atlantic and AgulhasCurrent region. The provenance of terrigenous sedi-ments is identified using the radiogenic isotopecompositions of strontium (87Sr/86Sr) and neodymium(143Nd/144Nd), and rubidium, strontium, samarium andneodymium (Rb, Sr, Sm, Nd respectively) concentra-tions. We infer the transport pathways based on thespatial patterns of sediment provenance and flux.Comparison of Holocene and LGM data allowsdelineation of major differences in the regional surfaceand deep ocean circulation. The results strongly indicatea reduced Agulhas flux during the LGM.

2. Sample selection and methods

2.1. Sample selection and stratigraphy

For a detailed provenance and flux analyses,thirteen sediment cores were selected from the AgulhasCurrent region, the Cape Basin, and the nearby SouthAtlantic and Atlantic sector of the Southern Ocean(Fig. 1 and Table 1) for which the core tops areHolocene and the depths of the LGM are known.Samples were analyzed from core tops and from thedepth of the LGM (Table 1). A modern sample fromthe Orange River was also analyzed. Samples with

Table 1Core locations and the stratigraphic location of the LGM in each core

Region, Core Latitude Longitude Water

Agulhas Current1 VM19-214 −23.37 38.85 30922 RC17-69 −31.50 32.60 33803 VM14-77 −29.63 32.87 18184 RC11-86 −35.78 18.45 2829

Cape Basin5 VM19-240 −30.58 13.28 31036 RC13-229 −25.50 11.30 41917 RC13-227 −21.88 8.87 4301

Cape Basin/South Atlantic Current8 RC13-243 −36.90 0.33 4790

South Atlantic Current9 RC13-251 −42.52 11.67 4341

Antarctic Circumpolar Current10 RC11-77 −53.05 −16.45 409811 RC11-80 −46.75 −0.05 365612 RC12-289 −47.90 −23.70 448413 RC13-255 −50.58 2.90 3332

high concentrations of foraminifera were selected for14C analyses (Supplement 1).

A larger scale map of the 87Sr/86Sr ratios ofHolocene-aged terrigenous sediments (Fig. 3a) in theSouth Atlantic was generated using a large suite of coresfrom the Deep-Sea Sample Repository at Lamont-Doherty Earth Observatory (LDEO). Samples weretaken from within the top 10 cm of each core. A similarmap was created for the LGM (Fig. 3b), by samplingcores at the horizon established for the LGM based onpublished stratigraphies.

2.2. Sr and Nd isotope ratios and Rb, Sr, Sm and Ndconcentrations

Measurements were made on the <63 μm terrigenoussediment fraction. Samples were processed usingprocedures similar to previous studies from this lab[14,20]. We removed calcium carbonate using abuffered acetic acid leach (pH=5), ferromanganeseoxides using 0.02 M hydroxylamine HCl, and opal fromsamples south of 40° S using a 2 N sodium carbonatesolution [22]. Sample dissolution and separationchemistry were modified from the procedures describedin [14,20]. Aliquots comprising 10% of the dissolvedsamples were removed and spiked for isotope dilution.They were then allowed to equilibrate in 5 N HCl at

depth (m) Depth to LGM (cm) Reference(s)

40 [59]50 [60,61]29 [60,61]45 [60,61]

60 [50,61]80 [50,61]40 [61]

40 [62]

20 [62]

200 [63]45 [62]60 [62]320 [62]


∼120 °C for several hours before passing throughseparation chemistry.

Strontium and neodymium isotope ratios (Figs. 3–6and Supplement 2) were measured at LDEO by dynamicmulti-collection using a VG Sector 54-30 thermalionization mass spectrometer (TIMS). Concentrationsof Sr, Sm and Nd (Fig. 6 and Supplement 2) weremeasured by isotope dilution on a VG Axiom multi-collector inductively coupled plasma mass spectrometer(MC-ICP-MS) at LDEO. Concentrations of Rb weremeasured by isotope dilution on a VG PQ2 quadrupoleICP-MS at LDEO.

2.3. 230Thxs0 and 230Th-normalized sediment flux

One hundred milligram samples of bulk sedimentwere processed for the 230Thxs

0 measurements. Sampleswere spiked with 229Th and uranium-236 (236U) beforedissolution. The dissolution chemistry is similar to thatof Anderson and Fleer [23]. An iron hydroxide co-precipitation step was used to concentrate U and Thbefore separation using 5 ml AG1-X8 anion exchangeresin cleaned with dilute (∼1 N) HCl. Samples wereloaded in 8 N HNO3. Thorium was eluted first, using15 ml concentrated (10–12 N) HCl. U was then elutedwith 15 ml 0.12 N HCl. Isotope ratios of U and Th weremeasured simultaneously using the VG Axiom MC-ICP-MS at LDEO. Decay-corrected 230Thxs

0 activitiesand 230Th-normalized fluxes were calculated for eachsample (Supplement 3).

3. Results

3.1. 87Sr/86Sr and εNd of South Atlantic and CapeBasins sediments

Previous studies have demonstrated the validity ofusing Sr and Nd isotopic compositions (87Sr/86Sr andεNd) of terrigenous detritus to infer their provenance andrelate them to changes in ocean circulation patterns [24–28]. 87Sr/86Sr are sometimes subject to sorting effects, inwhich case they vary with grain size (reflecting variableRb/Sr in different types of minerals), and thus canpartially reflect the transport processes [24,27]. Ingeneral, Nd isotope ratios are less affected by sortingand more accurately reflect the average age of thecontinental sources [25–27,29], although it is possiblethat they vary with grain size as well. Sm–Nd depletedmantle model ages (TDM) are calculated using present-day depleted mantle values of 143Nd/144Nd=0.51315and 147Sm/144Nd=0.217 to describe the evolution of themantle complement of the continental crust [29,30], and

represent the average “crustal residence” age (time sinceextraction from the mantle) of the sediment sourceterrains.

South Atlantic Holocene-aged terrigenous sedi-ments display systematic geographical variationsspanning a large range of Sr isotope ratios from87Sr/86Sr=0.704 to 0.736 (Fig. 3a). The highest valuesare east of southern Africa in the Agulhas Currentregion, near the Archean Kaapvaal craton, and thelowest values are near southern South America, wherethere is a significant contribution from Patagonianvolcanic rocks. In the Southeast Atlantic, andparticularly in the Cape Basin, 87Sr/86Sr are inter-mediate between the two extremes. Within the CapeBasin, the highest 87Sr/86Sr are off the southeastAfrican coast and north of the Agulhas Ridge,whereas values are much lower in the center of theCape Basin. This regional relationship is consistentwith expected variations in the Nd isotope ratios (Fig.4) [given as εNd, the fractional deviation of143Nd/144Nd from the “bulk Earth” value of0.512638 [31] in parts per 104] and Sm–Nd modelages (Supplement 2) of Holocene terrigenous sedi-ments. That is, εNd are lowest under the AgulhasCurrent and highest in the southwest Atlantic, andmodel ages are oldest under the Agulhas Current andyoungest in the southwest Atlantic. Intermediatevalues of εNd and TDM are found in and around theCape Basin. The modern Orange River sample haslower 87Sr/86Sr, higher εNd and a younger Nd modelage than most of the Cape Basin samples.

For the LGM, the general map pattern is similar tothe Holocene, with the highest 87Sr/86Sr, lowest εNd,and oldest model ages east of Africa, and lower87Sr/86Sr, higher εNd, and younger model ages to thesouthwest (Figs. 3b, 4 and 5a). In general, LGM87Sr/86Sr are lower and εNd are higher than those fromthe Holocene in the same core, particularly in the CapeBasin. Consistent with this, high 87Sr/86Sr do not extendas far south during the LGM, as evidenced by theposition of the (orange) 87Sr/86Sr >0.725 contour line(Fig. 3a and b) and by the relative changes at each coresite (Fig. 5a). Holocene and LGM values are similaronly in the north-easternmost core under the AgulhasCurrent (#1, VM19-214, 23.37° S, 38.85° E, 3092 m)and in the Southern Ocean.

3.2. 230Th-normalized fluxes

230Thxs0 , defined as the initial (decay-corrected) 230Th

activity in excess of that which is in secular equilibriumwith its parent 234U in the sediment at the time of

Fig. 3. A compilation of Holocene (a) and Last Glacial Maximum (b) South Atlantic terrigenous 87Sr/86Sr. Data are from this study (reported insupplemental data Tables 2, 4 and 5) and from the literature [14,24,64,65]. Continental geology is from [66]. Symbols marking core locations arecolor coded according to their Sr isotopic compositions (see scale in center). Contour colors follow the same scaling.


deposition, is an important constant-flux proxy [32]. Ifvertical settling were the only source of sequestered230Th to the sediment, the flux of 230Thxs

0 would dependonly on water depth, resulting from the constantproduction rate of 230Th from radioactive decay of234U in the water column, and its efficient scavengingonto sinking particles. 230Th-normalized fluxes can becalculated for any sediment component if its massfraction (or weight percent) in the sediment and thewater depth is known. 230Th-normalized terrigenous

fluxes are used here to limit the possible scenariosresponsible for the changes in provenance.

For the Holocene, 230Th-normalized terrigenousfluxes are highest in the Agulhas Current region (Fig.5b), supporting the provenance-based interpretation ofthe Agulhas Current as a major source of sediment tothe Cape Basin. During the LGM, 230Th-normalizedterrigenous fluxes were generally higher in all regions,as might be expected due to the lower sea level, exposedcontinental shelves and increased glacial weathering.

Fig. 4. Terrigenous εNd vs.87Sr/86Sr (measured on the terrigenous fractions from sediment samples) from the Cape Basin, the South Atlantic, and the

Agulhas Current region, and a surface sample from the Orange River, which drains into the Cape Basin. Holocene samples are shown as filled insymbols, and LGM samples are open symbols. Symbol shapes and numbering is the same as for Fig. 1. The compositions of the three end-membersdefined in Table 2 are shown as stars, which are surrounded by grey shaded areas to represent the range in possible values and the acknowledgementthat there are uncertainties associated with them. The mixing curve drawn was calculated based on the end-member values in Table 2. Also shown aredust and loess compositions from South Africa [40,67].


The 230Th-normalized terrigenous flux is 2 to 7 timeshigher at the LGM compared to the Holocene for thesamples from the eastern Atlantic sector of the SouthernOcean (under the path of the ACC), and more thantwice the Holocene flux in the eastern South Atlanticbelow the SAC (Fig. 5b). East of Africa in VM19-214(#1), the northernmost core in the Agulhas Currentsource region, the LGM flux is about twice theHolocene flux. In the Cape Basin and in the southernend of the Agulhas Current region, the LGM flux is lessthan 40% higher than the Holocene flux. Thus therelative increase in 230Th-normalized terrigenous flux,comparing the LGM to the Holocene, is lowest for thecores downstream in the Agulhas Current region, and inthe Cape Basin.

4. Discussion

4.1. Sediment sources and mixing

The simplest scheme for representing the composi-tion of sediment near southern Africa is a binary mixtureof sediment carried by (1) the Agulhas Current and (2)the SAC or ACC. Southwest Atlantic sediments havelower 87Sr/86Sr and higher εNd than sediments from theAgulhas Current region. The Cape Basin data generally

fall between these two extremes, but they do not liedirectly on a mixing curve between them (Fig. 4),because within the Cape Basin, local sources influencethe composition as well. Thus, the simplest realisticmixing model defines three end-members as: (1) onewhich is generated in southeastern Africa and carried bythe Agulhas Current; (2) one that originates from SouthAmerica and West Antarctica and crosses the SouthAtlantic via the SAC and/or ACC; and (3) a local SouthAfrican end-member, which is delivered directly to theCape Basin by winds and rivers. We have defined theircompositions (Table 2) using the isotopic and concen-tration data from the cores analyzed for this study andliterature data. Each of the three end-members is itself amixture of various other, more ‘pure’ end-membersources, and we regard them as the compositions ofsediments entering the Cape Basin, rather than as purecontinental end-members.

The “Southwest Atlantic end-member” is a mixture ofthe South American and West Antarctic sources (c.f.[28]). The “Agulhas end-member” is a mixture ofsediments derived mostly from Archean [33,34] andProterozoic [35] terrains in eastern and central SouthAfrican sources, drained to the ocean by the Limpopoand Zambezi Rivers. The “Local end-member” is amixture of the various southern African source terrains

Fig. 5. a: Holocene and LGM terrigenous 87Sr/86Sr from the ocean floor south of Africa. Data sources are the same as for Fig. 3. Symbols markingcore locations are color coded according to their Holocene Sr isotopic compositions. The label boxes follow the same color scaling. The Holocene(top) and LGM (bottom) values for the terrigenous 87Sr/86Sr from our cores and for RC11-83 [20] are given next to each core. For most of the coresanalyzed for this study, the glacial value is the same as, or is lower than the Holocene value. The most extreme changes in terrigenous 87Sr/86Sr areobserved south and southwest of Africa. b: A compilation of Holocene and LGM 230Th-normalized terrigenous fluxes from the ocean floor south ofAfrica. Data are from this study (reported in Supplement 3) and from the literature [44,68]. Symbols marking core locations are color coded accordingto their Holocene 230Th-normalized terrigenous fluxes. The label boxes follow the same color scaling. The Holocene (top) and LGM (bottom) valuesfor the terrigenous 230Th-normalized fluxes are given next to each core.



with a large range in age and composition [36–40],including those drained by the Orange and Tugela Rivers(Fig. 1), many small rivers, as well as eolian sources.Parent–daughter element ratios, isotope ratios, and theinferred model ages (Supplement 2) show that thedifference in the end-member sources is predominantlyage, rather than lithologic composition. 147Sm/144Nd forall of the samples are within 0.109± 0.009, indicatingtypical continental sources. The Agulhas end-memberrepresents a predominantly older (TDM=2.0 Ga) con-tinental crust provenance than the Southwest Atlanticend-member (TDM=1.0 Ga). The Local end-memberrepresents a mixture of many sources with a large rangein ages and a complex geologic history.

The composition of the Orange River sampleanalyzed here (Figs. 3a and 4) is dissimilar to that ofthe Local end-member, having much lower 87Sr/86Srand εNd. This highlights two observations: (1) that theLocal end-member is not dominated by inputs from theOrange River and must have a significant contributionfrom smaller rivers and/or from eolian sources; and (2)there is significant flexibility in describing the composi-tion of the Local end-member. The Holocene coresample from the Tugela cone (VM14-77, #3) is notrepresentative of the Local end-member either. TheAgulhas Current passes over this core site, contributingsediment with high 87Sr/86Sr and low εNd, whilenortheast flowing bottom currents complicate themixing relationship by redistributing the sediments,and possibly adding material from the southwestAtlantic and Circum-Antarctic. The relatively low Srconcentrations of the three sedimentary end-members(Table 2) compared to the 350 ppm Sr concentrationestimated for average upper continental crust [30], areprobably the result of Sr loss during weathering andtransport [29,41,42]. The Local end-member has verylow concentrations of 40 ppm Sr and 10 ppm Nd, whichwe attribute to dilution by eolian quartz reflecting thearid environment.

Table 2End-member compositions

Region Sr(ppm)

87Sr/86Sr Nd(ppm)

143Nd/144Nd εNd

Agulhas 75 0.740 37 0.51177 −17South Atlantic 165 0.709 21 0.51238 −5Local 40 0.731 10 0.51233 −6

The end-member compositions were chosen so that the calculatedmixing curves between them produce fields that envelop all of theCape Basin data from both the Holocene and the LGM on five differentmixing plots (not all presented here): εNd vs.

87Sr/86Sr; 87Sr/86Sr vs. 1 /[Sr]; εNd vs. 1 / [Nd];

87Sr/86Sr vs. [Nd]/[Sr]; and εNd vs. [Sr]/[Nd].

4.2. Provenance of Cape Basin sediments

The pattern of 87Sr/86Sr in South Atlantic Holoceneterrigenous sediments (Fig. 3a) is similar to present daysurface ocean circulation (Fig. 1). The general trend is ofincreasing 87Sr/86Sr to the east, away from SouthAmerica, following the paths of the SAC and ACC.Close to Africa, there is a trend of decreasing 87Sr/86Sr tothe southwest, away from the source of the highest87Sr/86Sr in southeastern Africa, along the path of theAgulhas Current. This trend continues around the tip ofAfrica, following the Benguela Current flow to thenorthwest, and it also turns east south of Africa, followingthe Agulhas Retroflection. We therefore conclude that thesurface currents have an important effect on the distribu-tion of sediments in the South Atlantic.

The map pattern shows that the Agulhas Current isclearly the source of high 87Sr/86Sr and low εNd in theCape Basin (Fig. 3a). In addition, the strong surfacecurrents of the SAC and ACC are likely responsiblefor the long distance transport of fine-grained materialwith lower 87Sr/86Sr from South America clear acrossthe Atlantic Ocean. The SAC is at an appropriatelatitude for fine-grained material to rain out directlyover the Cape Basin. The ACC is the much larger andstronger current, making it a more likely sedimentsource, but it flows ~10° south of the Cape Basin (Fig.1). For material transported by the ACC to enter theCape Basin, it must be rained out and subsequentlybrought northward by the bottom water (AABW) flow.The bottom water flow within the Cape Basin finallyredistributes the sediment that was brought in fromdistant sources and mixes it with locally derivedterrigenous sediments. The observation that the OrangeRiver is not an important contributor to the composi-tion of Cape Basin sediment supports the predomi-nance of more distant sediment sources.

The general spatial patterns for the LGM data aresimilar to the Holocene (Fig. 3a and b), but most samplesaround southern Africa show shifts toward lower87Sr/86Sr and higher εNd. There are three major processesthat could have produced the observed changes incomposition: (1) sedimentary sorting resulting insystematic changes in grain size distribution; (2)climate-related changes in continental weathering,resulting in temporal changes in the composition ofone or more of the sedimentary end-members; and (3)temporal variations in the flux of material from one ormore end-members reaching the core sites. All three ofthese processes may contribute to some extent, but theprimary cause of both the spatial and temporal variabilityin 87Sr/86Sr and εNd is provenance.


While it is possible that some of the variability in87Sr/86Sr and εNd is due to sedimentary sorting, if sortingwere responsible, we would expect to see regionalcorrelations between parent–daughter ratios and isotoperatios. The data (Supplement 2) indicate very littlevariability in Sm/Nd, and show little evidence forcorrelations between Sm/Nd and Nd isotope ratios.This implies that the Nd isotope variability reflectsprovenance. The large-scale correlation between87Sr/86Sr and εNd (Fig. 4) must be due to provenancevariability as opposed to sedimentary sorting, given thebroad spatial scale (the entire South Atlantic). Addition-ally, for almost all of the cores, the LGM compositionshifts to lower 87Sr/86Sr and higher εNd (Figs. 3 and 5aand Supplement 2). We would not expect a temporalchange in sorting regime to be similar for so many of ourcore sites, given that they span a large range insedimentary and oceanographic settings and are influ-enced by a variety of ocean currents.

We also do not believe that changes in theweathering regime are an important factor for theobserved isotopic changes. It is well known that therehave been significant changes in the climatic regimes(especially aridity) over southern Africa on glacialtimescales [43]. If our sediment source regions haveseen shifts between wet and dry climate regimes, andconsequently between primarily chemical and physicalweathering, we might expect to see changes in theisotopic compositions of the material being weatheredfrom the continent. Our data do not support changes inthe composition of any of the three mixing end-members between the Holocene and the LGM. Thenorthernmost core from beneath the Agulhas Current,VM19-214 (#1) has almost identical 87Sr/86Sr and εNdin the Holocene and LGM (87Sr/86Sr=0.736 and 0.734and εNd=−15.5 and −14.8), which are appropriate asthe Agulhas end-member for both time-slices (Figs. 4,5a and 6). The southwest Atlantic data clusters around87Sr/86Sr=0.710 and εNd=−5.5, with no discernableshift between the Holocene and LGM (Figs. 4, 5a and6), thus this end-member also appears to be constant.The composition of the Local end-member was chosento envelop the Holocene data, and again, no change isnecessary to explain the data for the LGM. Since wesee no evidence for a Holocene to LGM change in thecompositions of either African-derived end-member,we conclude that weathering changes have notsignificantly influenced the compositions of oursediments. The primary control on the composition ofterrigenous sediment south of Africa seems to beprovenance variability, while variable sorting andweathering play minor, possibly insignificant roles.

During the LGM the samples from the Cape Basinare generally closer in composition to the SouthwestAtlantic end-member (Figs. 4 and 6), while those fromcores south and southeast of Africa show increasedproportions of both the Southwest Atlantic end-memberand the Local end-member (Fig. 6). We can use theisotopic and elemental data for each core (Supplement2) along with the compositions of the end-members(Table 2) to calculate the fraction of each end-memberthat contributes to each sediment sample. The resultsshow that during the LGM, Southeast Atlantic sedi-ments received a smaller proportion of their totalsediment input from the Agulhas source than they dotoday. The lower terrigenous 87Sr/86Sr ratios in RC11-83 and in other Cape Basin sediments at the LGM (Figs.2, 4 and 6) could be the result of either a decreasedglacial flux of the high 87Sr/86Sr Agulhas end-member[14] or an increased glacial flux of the low 87Sr/86SrSouthwest Atlantic end-member [20]. With the isotopicdata alone, these two scenarios are indistinguishable, asthe composition of a mixture depends only on therelative proportions of the end-members. The 230Th-normalized terrigenous flux data combined with theprovenance data can be used to distinguish thesescenarios. Table 3 gives the flux of each end-memberto each sample. In general, the LGM samples showlower fluxes of Locally derived and Agulhas derivedmaterials, while all but one core show higher LGMfluxes of South Atlantic derived material.

The 230Th-normalized terrigenous flux data (Fig. 5b),showing LGM fluxes up to 7 times higher in the Atlanticsector of the Southern Ocean, further indicate that thegreater relative contribution from distant SouthwestAtlantic sources at the LGM was due to an increase insupply of material from the Southwest Atlantic andCircum-Antarctic. Extremely high glacial terrigenousfluxes have been found by other authors in the Atlanticsector of the Southern Ocean and in the Scotia Sea, someup to 5 times greater than the Holocene [28,44,45]. Someof these sites were at water depths too shallow to havebeen influenced by AABW [28,44,45], so the enhancedsupply of lithogenic material during the LGM must havereflected, at least in part, greater sediment transport by theSAC and ACC. Provenance data from other sites in theCape Basin and Angola Basin [46,47] also point to astronger absolute input of clay-sized detritus from distantAntarctic and/or Patagonian sources carried northward tothe Cape Basin by deep currents during glacial periods,possibly large enough to overprint the signal of theregional background sedimentation. This seems to be thecase for the terrigenous 87Sr/86Sr record in RC11-83,which can be explained by an increased supply of

Table 3End-member fluxes to each core site

Core Holocene flux (g/cm2/ka) LGM flux (g/cm2/ka)

Agulhas S. Atlantic Local Agulhas S. Atlantic Local

Agulhas Current1 VM19-214 0.63 0.05 0.07 1.51 0.00 0.002 RC17-69 0.34 0.09 0.31 0.33 0.39 0.293 VM14-77 0.16 0.19 0.21 0.11 0.32 0.204 RC11-86 0.26 0.00 0.31 0.07 0.18 0.23

Cape Basin5 VM19-240 0.09 0.02 0.11 0.07 0.07 0.086 RC13-229 0.07 0.04 0.12 0.06 0.19 0.077 RC13-227 0.04 0.03 0.08 0.02 0.05 0.09

Cape Basin/South Atlantic Current8 RC13-243 0.02 0.14 0.06 0.02 0.23 0.18

South Atlantic Current9 RC13-251 0.01 0.08 0.02 0.01 0.26 0.24

Antarctic Circumpolar Current10 RC11-77 0.00 0.28 0.10 0.00 1.26 0.1511 RC11-80 0.00 0.11 0.03 0.00 0.22 0.1212 RC12-289 0.00 0.50 0.08 0.20 1.10 0.0013 RC13-255 0.00 0.20 0.00 0.00 0.95 0.43

End-member fluxes were calculated by multiplying the 230Th-normalized terrigenous fluxes from Supplement 3 by the mixing fraction of eachend-member. The mixing fractions were determined as follows, using the data in Supplement 2 and the end-member compositions in Table 2:fAgulhas= ((RSample

Nd ⁎CSampleNd −RLocal

Nd ⁎CLocalNd )⁎ (RSouth Atlantic

Sr ⁎CSouth AtlanticSr −RLocal

Sr ⁎CLocalSr )− (RSample

Sr ⁎CSampleSr −RLocal

Sr *CLocalSr )⁎ (RSouth Atlantic

Nd ⁎

CSouth AtlanticNd −RLocal

Nd ⁎CLocalNd ))/(RAgulhas

Nd ⁎CAgulhasNd −RLocal

Nd ⁎CLocalNd )⁎ (RSouth Atlantic


Sr ⁎CLocalSr )− (RAgulhas

Sr ⁎CAgulhasSr −RLocal

Sr ⁎

CLocalSr )⁎ (RSouth Atlantic

Nd ⁎CSouth AtlanticNd −RLocal

Nd ⁎CLocalNd )); fSouth Atlantic = ((RSample

Sr ⁎CSampleSr −RLocal

Sr ⁎CLocalSr )− fAgulhas⁎ (RAgulhas

Sr ⁎CAgulhasSr −RLocal

Sr ⁎

CLocalSr ))/(RSouth Atlantic


Sr ⁎CLocalSr ); fLocal =1− fAgulhas− fSouth Atlantic; where: f=mixing fraction; RNd=143Nd/144Nd; CNd=Nd

concentration in ppm; RSr =87Sr/86Sr; and CSr=Sr concentration in ppm.


material with lower 87Sr/86Sr during glacial times.Therefore, the terrigenous 87Sr/86Sr record from theCape Basin (Fig. 2) cannot be interpreted as a record ofchanges in the Agulhas Leakage over time. Otherstrategies must be employed to reconstruct past changesin Agulhas Leakage.

4.3. Reconciling SST and provenance evidence for coresin the southern Cape Basin

Rutberg et al. [20] rejected the hypothesis that the87Sr/86Sr variability in RC11-83 was caused by reducedAgulhas Leakage because of the mismatch between the87Sr/86Sr record in that core and proxies of sea surfacetemperature (SST) in nearby core TNO57-21 (41.13° S,7.82° E, 4981 m) between ∼20 ka and ∼40 ka (Fig. 7).Our interpretation requiring a third end-member toexplain the provenance variability can help to resolvethis apparent discrepancy. The SST record would not bea simple average of the SSTs of the incoming watermasses, but would also depend on the relative

concentrations of particles (alkenones or clay minerals)carrying the measured signals (alkenone-SST and87Sr/86Sr) for each of the three sediment sources. It isexpected that the Benguela upwelling region, due to itshigh productivity, has a much higher concentration oforganic material compared to the other sources, and thusshould dominate the alkenone-SST signal recorded indrift sediments. It is therefore not surprising that theTNO57-21 alkenone-SST record looks very much likethat of the Benguela upwelling region (Fig. 7) [48,49].In contrast, the variability in 87Sr/86Sr is driven mostlyby the balance between the Southwest Atlantic andAgulhas end-members because of their much higher Srconcentrations and more extreme isotope compositionscompared to the Local end-member. Accordingly, alikely mixing scenario could produce significantalkenone-derived SST changes with little to no changein 87Sr/86Sr. The dissimilarity of the alkenone-derivedSST recorded in TN057-21 with the 87Sr/86Sr in RC11-83 does not preclude them both from being the result ofmixing.

Fig. 6. 87Sr/86Sr vs. Nd/Sr of the terrigenous fractions of Holocene (top panel) And LGM (bottom panel) sediments. We use the elemental ratio, Nd/Srrather than 1 /Sr to normalize out the effects of diluting components in the sediment that may not have been removed by our leaching procedure.Symbols and numbering are the same as for previous figures. The three end-members listed in Table 2 are shown as stars with shaded fields drawnaround each end-member to show that they are not well-defined. Shaded arrows show the mixing trend of the samples around Africa.


4.4. Evidence for a reduced Agulhas Leakage duringthe LGM

At each site presently under the influence of theAgulhas Current (cores 2–7), except for the northern-most core VM19-214 (#1) representing the Agulhas

end-member itself, the relative contribution of Agulhasend-member sediment during the LGMwas less, and thecontribution of the Local end-member was greater,compared to the Holocene (Fig. 6). For the Holocene,these cores plot on a trend indicating mixing primarilybetween the Agulhas end-member and a composition

Fig. 7. (a) Terrigenous 87Sr/86Sr ratios for RC11-83 [20]. (b) Alkenone-derived SST for TNO57-21 [15]. (c) Alkenone-derived SST for cores in theBenguela upwelling region [49,69,70]. Marine Isotope Stages (MIS) are labeled and warm stages are highlighted in gray.


intermediate between the Southwest Atlantic and Localend-members (Fig. 6). During the LGM, the trend forthese sites shows a marked shift away from the Agulhasend-member, while the composition of the Agulhas end-member itself remains the same. The cores south andsoutheast of Africa (cores 2–4) have LGM compositionsmore similar to the Local end-member, and coreslocated southwest of Africa now plot along a trendthat reflects mixing between the Local end-member andthe distant Southwest Atlantic end-member.

Our 230Th-normalized fluxes can be used todiscriminate between a lower supply of Agulhas end-member sediment during the LGM versus a greatersupply of Local end-member sediment to account forthese observations. During the LGM the 230Th-normal-ized terrigenous flux to the northern Agulhas coreVM19-214 (#1), representing the Agulhas end-member,was twice that of the Holocene (Fig. 5b). From this weinfer that the sediment load carried by the glacialAgulhas Current was probably at least twice what it istoday. By contrast, at the downstream sites and in theCape Basin (cores 2–7), LGM fluxes of Agulhas end-member sediment were lower than during the Holocene(Table 3). Additionally, the glacial flux of Local materialto each of these cores was similar to or lower than theHolocene flux (Table 3). The greater relative influenceof the Local end-member composition during the LGMwas not caused by a large glacial flux of Local material.Therefore, the smaller relative Agulhas contribution tosediments south of Africa during the LGM can only be

explained by a lower supply of Agulhas end-membersediment. Building on this, if the Agulhas Currentcarried more sediment during the LGM than during theHolocene, but less Agulhas end-member sedimentreached sites south of Africa, then we can furtherconclude that, on average, less Agulhas Current waterwas making its way around Africa and into the SouthAtlantic Ocean during the LGM.

4.5. Ocean currents during the LGM

The provenance and flux data presented here,indicating a smaller glacial flux of Agulhas materialinto the Cape Basin (cores 5–7) at the LGM, impliesthat there was less leakage of Agulhas water into theAtlantic. There is substantial, but equivocal, evidencethat the Agulhas Leakage into the South Atlantic wasreduced during glacial periods [8–12,46,50,51]. Mostof the prior evidence is based on the abundances offloral and/or faunal species (e.g. G. Menardii[11,50,51], D. tetrathalamus [50]) or assemblages[8,9,12] that are presently associated with the AgulhasCurrent. In general, the data is consistent with reducedAgulhas Leakage during LGM, but there are alter-native explanations for almost every data set. Addingour terrigenous provenance and flux data to thepresent body of knowledge provides strong evidencethat there was less Agulhas Leakage during the LGM.

The smaller glacial flux of Agulhas material to thecores southeast of Africa (cores 2–4) requires that either


the glacial Agulhas Current was weaker than today or ithad a different flow path. If the Agulhas Current becamedetached from the coast before reaching 30° S, it couldexplain the predominance of Local end-member sedi-ments at cores 2 and 3 over those transported by theAgulhas Current during the LGM (Fig. 6). However,δ18O of planktonic foraminifera from cores between 27°S and 34° S suggests that the Agulhas Current occupieda similar flow path at the LGM [52]. If the AgulhasCurrent had ceased to flow over these core locations atany point in the last glacial cycle, the planktonicforaminifera living at the surface would have seen a vastreduction in water temperature, which should berecorded as a significant enrichment in the δ18O oftheir shells. The authors of that study [52] find no suchobvious enrichment above that which would beexpected from ice volume and a slight glacial coolingof the Agulhas Current. Therefore, they conclude thatAgulhas Current did not laterally shift its core locationor pathway significantly during the LGM. If this is true,then the only remaining explanation for the reduced fluxof Agulhas sediment to cores 2 and 3 (Fig. 6) is areduced volume of Agulhas transport at the LGM.

Previous studies of planktonic foraminifera assem-blages and δ18O in core RC17-69 (#2) also revealedevidence that the Agulhas Current was probably weakerduring glacial periods and stronger during the inter-glacials [7,53]. These findings were based on thedominance of a more sub-tropical species assemblageduring glacial periods compared to the tropical assem-blage that is dominant during the Holocene [7], as wellas the reconstructed temperature difference betweenRC17-69 and a core off the coast of Australia [53]. Amore recent study of a core site in the Indian sector ofthe Southern Ocean [54] finds greater advection ofanomalously warm alkenones during the LGM. Theauthors attribute the source of these alkenones to be theAgulhas Current, and offer two possible explanationsfor this observation. One is a more intense AgulhasCurrent at the LGM. The other is a more intenseAgulhas Return Current, due to reduced Leakage intothe Atlantic. This second scenario would not require thatthe Agulhas Current itself was stronger, and therefore isconsistent with our findings of a weaker AgulhasCurrent at the LGM.

Reduced Agulhas Leakage at the LGM could berelated to the Agulhas Current transport and/or thelocation of Retroflection. Modeling studies and recentobservations indicate that the location of the AgulhasRetroflection (and the magnitude of interoceanexchange) is heavily dependent on the volume flux ofthe Agulhas Current [55]. A stronger Agulhas Current

forces an early detachment from the South African coastand the retroflection occurs farther east, resulting in areduced leakage of Agulhas water into the Atlantic.Conversely, a weaker Agulhas Current allows themodeled retroflection to propagate farther west, leadingto increased Leakage. However, the evidence presentedhere, combined with the best paleoceanographic evi-dence in prior literature, points to a situation at the LGMwhere the Agulhas Current was weaker than today, andthere was reduced leakage of Agulhas water into theSouth Atlantic Ocean. This is inconsistent with themodel results described above, and calls for furtherinvestigation into the real controls on the AgulhasLeakage.

Our strategy of looking at changes in the flux ofterrigenous sediment from different sources can beapplied to a number of cores in the Retroflection regionin order to constrain the LGM to Holocene changes in theposition of the Retroflection. With this additionalinformation, we will better understand the forces drivingthe magnitude of Agulhas Leakage over the last20,000 yr. Whether changes in the leakage of IndianOcean water into the Atlantic had an effect on the globaloverturning circulation or on the formation of NADW isstill a matter of speculation, as no deep-sea core yet existsfor which there are records of both the Agulhas Leakageand the strength of the thermohaline circulation system.

5. Conclusions

This is the first study to combine provenance tracerswith 230Th-normalization, which together have greatpotential for tracing sediment transport and sourcehistory. Trace element and Sr–Nd isotope systematicsand 230Th-normalized fluxes of terrigenous sediment inthe Cape Basin show that distant continental sources areimportant contributors to Cape Basin sediments. Localsediment sources to the Cape Basin are derived from theOrange River, from smaller rivers and from eolian dust,but contributions from these are minimal, rather, surfaceocean currents play a dominant role in sedimenttransport. In particular, sediments from eastern SouthAfrica are brought to the Cape Basin via the AgulhasCurrent, and sediments from South America andAntarctica are carried across the Atlantic by the SACand the ACC. Bottom currents are important forlocalized redistribution of sediments within the CapeBasin, and possibly for northward transport of sedi-ments from the Southern Ocean.

Holocene Cape Basin sediments are mixtures ofAgulhas, Southwest Atlantic, and Local “end-member”sources, each a hybrid of multiple continental sources.


During the LGM, the 87Sr/86Sr and εNd of the end-members were not significantly different from theirrespective Holocene values, but the isotopic composi-tions of Cape Basin sediments were closer to theSouthwest Atlantic end-member. The increased con-tribution of sediments from South America and WestAntarctica during glacial times is a significantcomponent of the down-core variability in theterrigenous 87Sr/86Sr record of RC11-83, reported byRutberg et al. 2005 [20]. The increased sediment loadwas probably the result of an increase in glaciogenicsediment input from the Patagonian and Antarctic icesheets during the LGM, facilitated by a strengthenedACC.

East of the Cape Basin, Holocene sediments south ofAfrica show mixing trends with a large Agulhascomponent, while 87Sr/86Sr and Nd/Sr ratios imply amuch greater contribution from local southern Africansources during the LGM. Provenance and flux dataimply that less sediment from the upstream AgulhasCurrent sources reaches these sites during the LGM,despite a higher terrigenous flux in the Agulhas sourceregion. Together these data indicate a weaker glacialAgulhas Current, and a decreased Agulhas Leakage.The results of this study illustrate the potential for usingsediment transport patterns as proxies for past surfaceocean circulation in the region of the Agulhas Currentand Retroflection. Further work in this region will leadto better constraints on Agulhas transport and Leakage,and the magnitude of latitudinal shifts in the AgulhasRetroflection at the Last Glacial Maximum, and help usto establish the links between Agulhas transport andLeakage in the past.

Acknowledgements

We thank C. Class for arranging the collection of theOrange River sediment sample. We also thank A.Gordon, W. Broecker and T. van de Flierdt for theirhelpful discussions leading to the final version of thiswork. This study was supported by NSF grants OCE 98-09253 and OCE 00-96427 to S.L.G. and S.R.H., an NSFGraduate Research Fellowship to A.M.F. and by agrants/cooperative agreement from the National Oceanicand Atmospheric Administration. The views expressedherein are those of the authors and do not necessarilyreflect the views of NOAA or any of its sub-agencies.Samples used in this project were provided by theLamont-Doherty Earth Observatory Deep-Sea SampleRepository, supported by the NSF (grant OCE 00-02380) and the Office of Naval Research (grant N00014-02-1-0073). This is LDEO Contribution XXXX.

Appendix A. Supplementary data

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.epsl.2006.07.002 [71–75].

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Documents

Reduced Agulhas Leakage during the Last Glacial Maximum inferred from an integrated provenance and flux study